Earth exploration



Dec. 17, 1940. J LEw|s 2,225,248

EARTH EXPLORATION Filed Jan. 5,- 1938 PERMEABILlTY MILLIDARCYS .7 FLUID INDEX 2500 2000 1500 1000 500 O I0 20 30 40 50 60 L r l l l I POROSITY on 1, VOLUME 50 40 30 20 IO 0 0 2O INVENTOR James A. LELULS ATTORN Patented Dec. 17, 194 0.

PATENT OFFICE EARTH EXPLORATION James A. Lewis and William L. liorner, Dallas, Tex., assignors to Core Laboratories, Inc., Dallas, Tex., a corporation of Delaware Application January 3, 1938, Serial No. 183,019

2 Claims.

This invention relates to a method of determining the probable productivity of oil sands from analyses of characteristics of core samples.

In drilling oil wells in fields containing gas under pressure the well during drilling is kept flooded with water or a water-mud suspension of sufllcient density to create a hydrostatic pressure suflicient to prevent the gas from escaping and the well from blowing. After the drilling is completed, the well is sealed off except for the layer or layers of sand at the horizon from which oil or gas, or both, are to be taken. But if the engineers in charge of the drilling have made a mistake as to the expected production from the horizon left exposed and the well instead of producing oil, produces water, or nothing at all, then by various relatively expensive operations other horizons may be opened up, or drilling is recommenced and the well is sunk to a greater depth in the hope of finding more productive sands.

Hence in drilling a well there is need for an accurate survey of the sand formations through which the well is drilled as regards the probable productivity of the sands. By productivity is meant whether sand will produce nothing, gas, gas and water, gas and oil, or gas, oil and water, and the relative amounts of each such fluid. Also, the quality of the oil produced, if any, is important because production of relatively small quantities of a better grade oil might prove more profitable than production of larger quantities of less valuable oil.

To this end during the drilling operation it is customary to cut cores from sands through which the well is drilled and which are thought likely to be oil producing. These cores have in the past generally been given only a cursory examination in so far as determining the producing characteristics of sands, because of the fact that a lot of time has been required for analyzing the cores. Another reason that these cores have been given only a cursory examination is that when the cores are cut from sands containing gas under pressure, the core samples, in being brought to the surface, lose this pressure so that the contents of the pores of the core samples at the surface are not in the same condition as existed in the undisturbed sand at he well bottom. Furthermore, during the cutting of the cores they are constantly being washed by the drilling mud being circulated through the well.

As a result, the core samples are generally analyzed only by smelling them to see if they have oil, by blowing through them to see if they are permeable, and by sucking them to see if they have gas and salt water. And the results from such an analysis are followed only when they are definitely positive. And as often as not guesses made from such analysis prove wrong.

Thus, although cores give the best and most knowledge about the condition of substrata sands, because no satisfactory methods of analysis or discovery of critical characteristics have been made, other methods of surveying the sands through which an oil well is drilled have been developed.

tive of oil-bearing sand. But at best this type of survey is general in nature, and does not tell the relative proportions of oil, gas and water that will be produced. Such a test is made generally as a supplement to coring to aid in telling whether or not sands possible of production have been passed through without coring or proper core analyses.

Another test used is the so-called drill stem test which is used when the driller feels his well may have reached oil-producing sand. In this test an actual sample of the fluid that sand at the well bottom produces is obtained. But this test is expensive and time-consuming if it gives negative results, and for various reasons does not give an indication of the rate of production.

Thus,.although these tests now in commercial use are helpful where a well has been drilled without coring, or without adequate analyses of cores taken from the well, the tests do not take 2 x assume portant characteristics of cores as rapidly as the i cores are taken from a well.

We have discovered a vital relationship between the probable production ,characteristics of the sand analyzed and certain of the characteristics obtained from the analysis. And in accordance with the presentinvention thisdiscovery is used in a method of estimating the probable .producing characteristics of sand as they are being, cored. More specifically, the method embodying the present invention involves the steps of analyzing the coresample to obtain as,

an index, the ratio of the percent free gas to the percent free water of the; core. By free gas is meant the pore space occupied by gas in the sample asit is taken from the well. By free as water is meant that connate water and any drilling mud-water which may have entered the pores during the cutting of thewcore and the bringing of the core to the surface. Then this index is compared with stable showing the type so of oil production that can be expected for different values of the index.

In addition the permeability and the oil saturation of the core sample are determined because of course no oil would be produced if no oil were present'in thesample and no oil would be produced unless there :is sumcient permeability to enable the oil to flow through the sand. From the analyses. an exploration chart of the sands through which thewell is drilled may be made, as

der pressure (often as high as 300 atmospheres),

that a relationship exists betweenthe ratio of the free gas content per unit volume of core and the "free water content per unit volume of core.

ea By free gas content is meant the pore space of the core filled with gas at the time it is analyzed at the surface of the well. By'free water content is meant the ,connate water present in the core plus any drilling mud-water which may have seeped go into the core during coring and the raising of r the core from the sand in which it was cut to the surfface of the well, and the probable producing characteristics of the core. This relationship can be used as an index to indicate whether the sand as will produce nothing, gas, oil and water, or oil,

water and gas.

The analysis by which such an index is determined is described in the said Homer application and may be made rapidly and conveniently at the well being drilled.

The graph, or exploration graph, shown in the drawing is illustrative of the kindof chart that may be obtained. when the readings on the chart are interpreted in the light of the following I interpretation in the table, the exploration chart indicates the type of fluid production to be expected from the sands from which the cores analyzed were taken. The ranges given in the table may vary wlththe different fields but the variation may be readily ascertained in a new field by noting production curves of a few typical wells surveyed in accordance with this invention.

Table content (oonnate+ m) Above an Oil.

0-40 0-76 Above The, above table may beused as axstandard for comparisomfor example, for the Gulf Coast and oil fields having similar producing characteristics The columns headed Pore Contents, in i all except doubtful cases, may be determined by the usualmethod of smelling and tasting but in doubtful cases must be analysed for oil and water. The divisioninto fcores of smaller diameter" and "cores of larger diameter has reference to cores 1 cut with the different kinds of coringapparatus now available in the field. The so-called "wireline. coring apparatuscuts a core of smaller diameter than the cores cut by the conventional coring apparatus in which the entire drill and drill pipe is'raised to remove the core. We have found that the cores of smaller diameters may in some instances be flushed during the drilling operation to a greater extent than the cores of larger, diameter, thuslpermitting original fluid contentsof the cores to be removed.x

In interpreting a chart such as shown in the drawing, we have observed that any real decrease in the index throughout an oil zone usually means water will be produced from that point. In other words, a most favorable oil sand is shown by an index which is well sustained throughout the section and which does not show casual irregularities on the graph.

As above pointed out and as shown in the drawing. the index should be used in connection witha permeability determinationbecause even though a sand horizon be rich in oil and gas, if it is not suiiiciently permeable, the rate of oil productionmaybe so low as to makethe well unprofitable.

Theusual procedure for determining thepermeability is slow and is limited in its application to solid sands (in contradistinction to soft sands) so we prefertowuse the method of determining 1 whether or not a sand is suiiiciently permeable .such as is described in the above mentioned Homer application. As described in the said application, the method comprises noting the length of timerequired for a non-compressible fluid to enter and compress gas inthe pores in a core sample. If, for example, under a pressure of say 30 to 50 atmospheres the non-compressible liquid fills up the pores almost instantly, the sample has suiliciently high permeability for com- I mercial production. Likewise, if about 95% of the liquid under 50 atmospheres enters the pores in say less than one second the permeability'is sufliciently high for commercial production. But if the time required for filling the pores with a liquid under 50 atmospheres is in excess of five seconds, this is an indication of an unfavorable permeability, particularly in the Mid-Continent and Gulf Coast fields. Hence, although the graph as shown is plotted in terms of millidarcies the permeability could be plotted in terms of permeability satisfactory or unsatisfactory to commercial production.

A rapid and satisfactory method of determining the gas content per unit volume of a sample of solid or soft sands is described in the above- 'mentioned Horner application. This method comprises measuring the volume of a sample of the cores by liquid-displacement and in such a way that the free gas content of the sample is not interfered with, and subsequently measuring the volume of space occupied by the gas. This may be accomplished by compressing the as with a non-compressible liquid, such as mer cury, in which the sample is immersed, by forcing the liquid into the pores to supplant the gas, and measuring the volume of liquid thus required. The amount of the liquid is then translated into percent gas per unit volume of sand.

By measuring the rapidity with which the noncompressible liquid under constant pressure enters the pores, an indication is obtained of the permeability of the sample which indications may be used as above discussed.

Of course each sand to produce oil must be analyzed for water to obtain the gas-water ratio of the core. In doubtful cases too the core must be analyzed for oil. A preferred method of analyzing the cores for these characteristics is described in the aforesaid Horner application.

Core samples are weighed and put in a still and raised to a temperature suiiicient to vaporize and distill off all the liquids present in the pores. The vapors thus driven off are cooled and condensed and measured. The amount of water distilled is plotted against time because the free water and drilling water comes off first and considerable time elapses before the water of crystallization comes over. After the distillation the amount of oil and the amount of water per unit volume of core sample by volume is obtained by dividing the results obtained by the weight of the core sample divided by the density previously determined during the free gas determination.

By dividing the results obtained from the gas content analysis by the water saturation analysis the index above mentioned is obtained. This index is then plotted as shown in right portion of the drawing and interpreted in the light of the information given in the table.

By adding together the amount of water, the amount of core space occupied by gas and the amount of oil present per unit volume of core sample, the pore space per unit volume is obtained and may be plotted as shown in the drawing.

We claim:

1. The method of measuring the oil-gas-water producing characteristics of substrata sands containing naturally imprisoned gas comprising the steps of cutting successive core samples from the sands, lifting such core samples uncased from the well, testing the permeability of the successive core samples to determine whether the sands are sufiiciently permeable for practical production purposes and recording the results, testing the oil saturation of the successive core samples in terms of the amount of oil per unit volume of core sample to determine whether the sands are sufliciently oil-bearing for practical production purposes and recording the results, measuring the free gas content of the successive core samples in terms of the amount of free gas per unit volume of core sample and recording the results, measuring the total water content of the successive core samples in terms of the amount of water per unit volume of core sample and recording the results, dividing the results of the gas measurements by the results of the water measurements, and converting the quotients of said divisions into fluid production indices by co-relating said quotients with the known fluid production indices of wells the producing performance of which in relation to their indices has been previously demonstrated.

2. The method of measuring the oil-gas-water producing characteristics of substrata sands containing naturally imprisoned gas comprising the steps of cutting successive core samples from the sands, lifting such core samples uncased from the well, testing the permeability of the successive core samples to determine whether the sands are sufliciently permeable for practical production purposes and recording the results, testing the oil saturation of the successive core samples in terms of the amount of oil per unit volume of core sample to determine whether the sands are sufiiciently oil-bearing for practical production purposes and recording the results, measuring the free gas content of the successive core samples in terms of the amount of free gas per unit volume of core sample and recording the results, measuring the total water content of the successive core samples in terms of the amount of water per unit volume of core sample and recording the results, dividing the results of the gas measurements by the results of the water easurements, and converting the quotients of said divisions into fluid production indices by co-relating said quotients with the ranges of known fluid production indices of wells the producing performance of which as to the production of water and oil in relation to said index ranges has been previously demonstrated.

JAMES A. LEWIS. WILLIAM L. HORNER. 

